gf friend tried to break us up....

so my gf is 18 and i'm 18 as well she has a friend which is like 30+ and creates comics and stuff on a forum...doesn't do much else...he told my gf I was just using her the first time...and tried to get her to believe....called me worthless and insulted me along other stuff...said I was bad and shouldn't be with her...my gf was upset each time...and it caused problems...she told him off like for 5 mins then that was it....so she protected me but not much....just went on speaking to him after those 5 mins like it was nothing....I confronted saying if he says anything again i'm going to speak to him because I value this relationship...and don't want him to ruin it...she actually questioned some of the stuff he said...and she was asking me questions and stuff...it caused big problems...she said I won't let you because I don't want to lose him as a friend...I was like how can he say that in the first place...I mean the guy is 30+ goes out with yes...kids..under the age of 18+ and his wife left him...he makes comics for a game on the forums most his time...but now whenever my gf speaks to him...I kinda get annoyed, I don't even want to hear about the guy...but my gf tells me anyway and doesn't see why i'm upset...then the other day just like I thought he started saying stuff again...any idea what I can do??